The present invention relates to a method of stabilising frequency and mode of a tuneable laser.
Tuneable semiconductor lasers include several different sections through which current is injected, these sections typically being three or four in number. The wavelength, power and mode purity of the lasers can be controlled by adjusting the current in the different sections. Mode purity implies that the laser is at an operation point, i.e. in a combination of the three or four injected drive currents that is characterised by the laser being distanced from a combination of the drive currents with which mode jumps occur and where lasering is stable and sidemode suppression is high.
Special requirements are required for different applications with respect to controlling wavelength. In the case of telecommunications applications, it is necessary that the laser is able to retain its wavelength to a very high degree of accuracy and over a long period of time, after having set the drive currents and the temperature. A typical accuracy is 0.01 nanometer while a typical time period is 20 years.
In order to be able to control the laser, it is necessary to map the behaviour of the laser as a function of the different drive currents. This is necessary after manufacture but prior to using the laser.
It is also highly desirable to be able to lock the wavelength of a laser and have control over the mode in which the laser operates, so that said laser will operate as intended over a long period of time. By mode control is meant optimisation of the laser operation point in operation, either continuously or at regular intervals, so as to eliminate the risk of a mode jump to some other cavity mode. Furthermore, it would be very beneficial if lasers could be automatically compensated for degradation in operation.
Several methods of mobilising the frequency and mode of a tuneable laser are known to the art. Several of these methods involve adjusting a current through a laser section so that the laser will continue to lase at the right frequency, while adjusting the currents through other laser sections while seeking a maximum or minimum in some measurable function, such as the laser output power.
Swedish Patent Specification No. 9900537-3 describes a method of wavelength locking and mode monitoring a tuneable laser. In this method, as with other methods, the laser is controlled at an operation point which lies at an extreme point on the measurable functions.
Certain functions have no usable extreme point. For instance, the output power of a GCSR laser may have a sawtooth configuration. Consequently, no extreme value is reached before the laser jumps to the next mode.
It is often found that the best laser operation point does not lie on a usable extreme point of the measurable functions, since such points often mean that the laser operates close to a mode jump.
Consequently, it is beneficial to lock control of the laser at, for instance, a given gradient of a curve, such as the curve of the output power as a function of reflector current. However, this causes a problem when the laser degrades, since such curves become xe2x80x9cstretchedxe2x80x9d as the laser ages, therewith changing the gradient of the curve.
The present invention solves the problem associated with the use of values that are not the extreme values of measurable magnitudes for controlling a laser that degrades.
The present invention thus relates to a method of frequency and mode stabilising a tuneable laser that has at least three sections, such as a Bragg laser or a GCSR laser, where measurable magnitudes are measured, said laser having been characterised with respect to a number of operation points, wherein the values of the measurable magnitudes are stored in a microprocessor or corresponding device, and wherein the method is characterised by causing the values of one or more of the measurable magnitudes to be non-extreme values; and is further characterised in that the measurable magnitudes include the back power in addition to the front power and frequency; and in that control currents for the gain section, phase section and reflector section of the laser, and when applicable also the coupler section, are caused to be controlled on the basis of the values measured for the measurable magnitudes while keeping both the front power and the back power constant.